A known cartridge contains developer therein and may also hold a developing roller therein. The developer in the cartridge may deteriorate before use, for example, due to moisture over a long period of storage. One solution to this problem is to keep developer in a sealed separate container, and dispose the sealed container in a casing of the cartridge. A known developer cartridge includes a casing and a baglike developer container that is disposed in the casing and contains developer. The developer container has an outlet sealed by a film. Removing the film causes the outlet to be open, allowing the developer to flow out of the developer container through the outlet.